The invention described herein arose in the course of, or under, Contract No. DE-AC08-83NV10282 between the United States Department of Energy and EG&G Energy Measurements, Incorporated.
Many commercial ground radar systems employ baseband video pulse radar technology developed in the 1960s. This technology inherently has several disadvantages associated with it. One would expect the transmit and receive signal pulses of the radar to be of similar shape and size. Unfortunately, due to antenna characteristics, soil attenuation properties, and the location of targets in the antenna's near field, a ringing effect is produced in the radar return. This ringing phenomena can be very complex and extremely difficult to decorrelate when trying to extract relevant information from the return signal. In addition, to achieve adequate resolution, the system must use very fast pulses. This places a finite limit on the average power, which in turn reduces the signal to noise ratio of such prior art systems. Another disadvantage in some of these prior art systems is the lack of phase information which, if available, could yield an improvement in the radar return signal through the use of digital signal processing techniques.
Among the more difficult problems which have been identified in some prior art systems is the unutilized low frequency components of the video pulse signal, the lack of coherence, and the inherently low average power of such systems. In addition, the difficulty of building high dynamic range sampling hardware, the lack of a broadband, high isolation, fast transmit/receive switch, and the impossible task of building an antenna to radiate the entire signal bandwidth, indicated the need for an entirely different approach to providing a radar system for detecting anomalies in geophysical media.
The deficiencies of commercial radar systems has also been noted by others in the radar field. For example, Fowler et al. U.S. Pat. No. 4,218,678 discloses an earth penetrating radar system having a transmitter section and a receiver section. In the transmitter portion, a digital signal from a microprocessor and a base periodic signal from a master oscillator are both fed to a frequency synthesizer where the signals are multiplied to produce a base reference signal which passes through an attenuator control and an amplifier to a transmitter antenna. Stepped frequency signals making up the Fourier frequency spectrum of the desired synthetic radar pulse are transmitted. In the receiver portion, a digital signal from the microprocessor and a base periodic signal from the master oscillator are both fed to a second frequency synthesizer where the signals are also multiplied to produce a base reference signal. The second synthesizer also includes a quadrature circuit wherein the base reference signal is converted into an in-phase reference signal and a quadrature reference signal. An incoming signal passes from a receiving antenna to an RF amplifier whose output is mixed with both the in-phase and quadrature reference signals to provide both in-phase (I) and quadrature (Q) output signals. The I and Q output signals are then digitized and recorded for each frequency, together with the frequency, until all frequencies have been transmitted and the return signals received at which time the time trace can be reconstructed by inversely transforming the I and Q values.
Stamm U.S. Pat. No. 4,381,544 describes a serial survey technique wherein microwave pulses of several frequencies are radiated to the ground from an antenna on an airborne platform. Part of each radiated pulse penetrates the ground and is absorbed or scattered and reflected by changes in the subsurface dielectric properties at the interfaces between materials having different dielectric properties. A detector also mounted on the airborne platform senses the reflected signals and has an empirically determined set of reflection criteria for each material interface.
Kyriakos U.S. Pat. No. 4,435,708 discloses a radio altimeter which uses a triangular modulating waveform for a frequency modulated transmitter. Digital means, synchronized with the triangle wave generator, produces a count gate which is at a high logic level during most of the linear portion of the triangle wave period, and which is at a low level during the portion of the period of the triangle wave near the wave peaks. The count gate and the beat frequency, produced by mixing transmitted and received signals, are applied to logic means which modifies the duration of the high level state of the count gate to produce a derived count gate having a high logic level always of such duration as to equal an integral number of cycles of beat frequency signal. The derived count gate is then used to control a beat frequency counter and a precision clock counter. The outputs of these counters are then arithmetically processed to yield digital altitude information free of step error.
Fowler et al. U.S. Pat. No. 4,504,833 discloses a system similar to that disclosed in Fowler et al. U.S. Pat. No. 4,218,678, except that in the receiver portion of the system, the digital signal from the microprocessor and the base periodic signal from the master oscillator are both fed to an offset synthesizer, while only the base periodic signal is fed to a quadrature circuit. The output of the offset synthesizer is mixed with the incoming signal from the receiver antenna and the RF amplifier in a receiver mixer whose output is then fed to a power divider where the signal is fed to both a first and second mixer. The respective in-phase and quadrature signals are also fed to the first and second mixers to provide in-phase and quadrature output signals.
Collins U.S. Pat. No. 4,620,192 describes a radar system wherein a signal from a master oscillator is mixed with a voltage controlled oscillator and the resulting signal is then passed through a filter before being amplified and sent to the transmitter antenna. A portion of the signal being transmitted is also coupled to the receiver to provide a local oscillator signal which is an undelayed replica of the transmitted signal. The incoming signal at the receiver antenna is split and sent to two mixers. The undelayed replica signal is also split and sent to the two mixers, after changing the phase of one of the signals by 90.degree.. The signals are heterodyned in each mixer to respectively provide in-phase and quadrature output signals from the mixers. The signals are then sent to a notch filter which will attenuate ground signals and pass any target return signals centered on a Doppler shift frequency differing substantially from zero frequency. The filtered signals are then digitized and passed to a digital signal processor which includes a digital correlator, an FFT signal processor, a magnitude processor, a memory, and a constant false alarm rate (CFAR) processor. The digital signal processor converts the digitized time domain data into a range/Doppler map and to report CFAR threshold crossings with the map to a digital computer.
Vacanti U.S. Pat. No. 4,670,753 describes a dual channel radar system wherein a transmitted FM signal, which is circularly polarized in one direction, sweeps a predetermined frequency range. The return signal, which is polarized in both directions, is received by the same antenna and the received reflections are mixed with samples of the transmitted signal to produce baseband frequency signals on two channels representing, respectively right and left-circularly polarized reflections. The signals are processed by an FFT element to produce digitized I and Q output signals for each channel. The minimum power measurements for each channel are determined and then compared in order to locate targets in the target area.
However, despite these attempts to improve upon existing commercial radar systems, there still remains a need for providing a radar system which will distribute the transmitted signal power requirements in a manner which will provide an increase in the systems overall average power, resulting in a less complicated, more reliable radar, as well as providing an increase in the system signal to noise ratio. In addition there remains a need for a radar system which is capable of preserving the phase information of the radar return signal and which will take full advantage of digital signal processing techniques which can provide an increase in the target detection ability of the system. Such a system should also provide for more efficient use of transmitted power by generating signals covering only the frequency range in which a practical antenna radiates most efficiently.